1. Field of the Invention
This invention relates in general to stretching fabric on frames and in particular to retensioning fabric on frames as used in screen printing.
2. Prior Art
Screen printing involves the use of stretched fabric that is attached to a frame. Desirable attributes of screens are 1) that the fabric is very tightly stretched to a consistent tension, 2) that the fabric is held in place on the frame, and 3) that the fabric after it is coated with emulsion and has had an art pattern exposed onto the emulsion can be released from the frame with the art pattern intact and put into storage to be later restretched on the frame so as to accurately duplicate the tension level previously attained and to accurately match the placement of the art pattern in relation to the coordinates of the frame with the position previously attained. It is also desirable that this process be achieved quickly and easily and that fabric is not wasted. The oldest method of stretching fabric is with clamps that are pulled by hand, pneumatic pressure, or mechanically. Tension meters are used to determine the tightness of the screen fabric and adjustments are made to either loosen or tighten the fabric in order to approach a prescribed tension level. Once the tension level is reached, the fabric is glued or stapled to the frame. Once this is done, the fabric cannot be released from the frame without destroying the fabric. These methods do not solve the problem of restretching the fabric or address efficient and convenient storage.
I have observed that in one field of screen printing, i.e., that of printing on glass cylinders, in which relatively loose fitting stainless steel fabric is used, the fabric has border strips of extruded plastic along one axis that cup over flanges on the outside of the frame thus holding the fabric in place on two sides only. The other two edges of the fabric have border strips of metal that are crimped on to the stainless steel fabric and bolted to the frame. This construction is specifically designed to allow electric current to flow through the screen and heat the screen to melt a thermoplastic printing ink. It was observed that there are no means in this construction to significantly tighten the fabric. The two edges of the fabric with the extruded plastic border strips can only be loosely pulled by hand and cupped over the frame flanges. The construction of the plastic borders does not provide a means for attaching a clamp or any type of stretching device to pull the fabric tight enough to meet the tension requirements of most screen printers. Also, there is no indexing of the extruded plastic border strips along the frame flanges which means that the plastic borders, which are shorter than the flange, could vary in their placement along the flanges. Along the other axis at the other two edges, the metal border strips were observed to be crimped onto the stainless steel fabric and had holes through which they were bolted to the frame. With more than just a minimal tension along this axis, the fabric would tear loose, especially polyester fabric.
This device allows the fabric to be loosely stretched, released, and loosely restretched with an art pattern only approximating its original position in relation to the coordinates of the frame.
Widely used today are draw bar frames U.S. Pat. Nos. 3,482,343 and 3,553,862 and roller frames U.S. Pat. Nos. 3,908,293 and 4,345,390. The draw bar frame incorporates pulling clamps as part of the frame and roller frames allow the fabric to be rolled tightly by means of rollers that form the four sides of a frame. These devices solved the problem of how to stretch fabric, release it for storage, and restretch the same fabric at a later time. However, it still required the trial and error method of reaching a prescribed tension level with the use of a tension meter and there still was no way of restretching an art pattern back to its original position in relation to the coordinates of the frame.
In U.S. Pat. No. 3,991,677 of 1976 by V. H. Barnes entitled "Printing Screen and Tensioning Means" is described a frame structure with a stretching mechanism incorporated into the frame structure. Also described is a screen with border strips that attach to tension bars of the stretching mechanism. In U.S. Pat. No. 3,211,089 of 1962 by Elmar Messerschmitt entitled "Screen Printing Screen" is described a frame structure with a stretching mechanism incorporated into the frame. The Messerschmitt invention uses a continuous border around the fabric which attaches to an element of the frame. The frame expands telescopically thereby stretching the fabric. U.S. Pat. No. 3,416,445 of 1965 by T. H. Krueger entitled "Screen Stencil with Separate Border Strips" describes various border strips, fabric, and frames. One embodiment of this invention has two adjacent sides of the fabric with border strips attached to a frame on studs whereas the other two opposite sides are pulled outward by flexible straps pinned to the border strips and wrapping around rotating crank shafts that are part of the frame. These flexible straps are all that hold these two border strips in place on this frame. Another embodiment of this invention features L shaped border strips. A third embodiment features a rigid continuous border that cannot be stretched and is fundamentally a frame attached to another frame.
The Barnes, Messerschmitt, and Krueger inventions require stretching mechanisms that are incorporated into the frames. The stretching mechanisms have moving parts which, like draw-bar frames and roller frames, are added expenses in the construction of the frames and add extra weight to the frames. These stretching mechanisms with their moving parts are exposed to the every day spraying, washing, and rinsing of the screens with water, cleaning compounds, solvents, and inks. They are subjected to the wear and tear of shop operations which usually includes a significant amount of mechanical shock. These parts, of course, depreciate under these severe conditions.
These three inventions do not utilize locking mechanisms that secure all four fabric edges into definite fixed positions on the frames. The Barne's invention does not have any pins, studs, abutments, etc. to fix the exact lateral location of the border strips on the tension bars. The Messerschmitt invention's continuous border is of a flexible material that stretches and is therefore unreliable as an aligning feature, particularly after repeated uses. The Krueger invention utilizes studs on two sides only to fix the locations of two unmoveable border strips. These two border strips do not at any time move to a more outward position on the frame. The other two border strips are attached to flexible straps which are unreliable as aligning features. Also, by only outwardly moving two border strips, the corner area between the stationary strips is under very little tension and the area between the two outwardly moved border strips is under extremely high tension. The tension throughout the fabric, because of the stretching method employed by this invention, is very inconsistent. This tension inconsistency in the fabric similarly exists in the L shaped border strips embodiment of this invention.
In none of the above cited references is there an inflexible aligning feature providing reliable alignment for a border strip that is moved to a more outward position. In none of the above cited references is there a discussion, object, or claim of controlling the stretch distances of the fabric along the X and Y axis by establishment of exact start and stop positions of the border strips so as to effect precalculated stretch distances for fabric of precalculated size and shape.
Whatever the precise merits, features, and advantages of the above cited references, none of them achieves or fulfills the purposes of the interlocking screen fabric, border strips and frames of the present invention.